No brakes would be fine for shorter hauls and lower speeds - even with 3500 lbs back there. I'd be a little more hesitant at highway speeds, and NEVER in hills / mountains.
I pull a single axle boat trailer (4500lb trailer). Yes it has surge brakes. However, I had an issue with an axle seal (Ranger trailer - oil filled hubs) that was leaking all over the disc - still stopped just fine (oil all over the disc sort of precludes braking on that side).
Your biggest issue wouldn't be stopping, it would be repetitive stopping - as in brake fade due to heat. There's enough clamping force and surface area that the brakes would do the job hauling all that mass down. The question is how hot would it get and would fade set in (from pads too hot, fluid starting to boil, etc) before the vehicle was stopped from higher speeds.
Also, depending on how much you tow and how frequent / hard you brake while towing, expect increased brake wear.
Now, all that said - I'm not sure what you currently tow with, but the brakes on a Touareg are FAR better than most everything else on the market. If you look at the brakes on a Chevy / Ford / Ram pickup or pickup based SUV, you will notice the discs are smaller - much smaller in many cases (less surface area) and most of them use a multi-piece caliper with piston(s) on one side only. The Touareg uses a one piece (solid) caliper with pistons on both sides. The result is a braking system that is able to generate more force, spread evenly across the surface of the pads. Look no further than any race car and you will see a similar set up.
BTW - I pulled my boat with my old V6 Touareg and it pulled great. There is a substantial difference in torque in the TDI motor, and my current Touareg pulls my boat almost effortlessly.
Good luck!